Conventionally, the phenolic novolak is generally produced by switching from a homogeneous reaction of a phenol and an aldehyde in the presence of an acid catalyst to a heterogeneous reaction starting from an emulsification phenomenon, further pursuing the reaction, checking the production of an initial condensate having a desired degree of condensation before terminating the condensation reaction, and conducting the reduction of low-molecular-weight components and dehydration in the initial condensate by vacuum concentration. And, the phenolic novolak has relatively favorable electric properties and also excels in heat resistance and flame retardance. Therefore, it is extensively used as a base resin for production of an epoxy resin which is used for, for example, electric and electronic materials, semiconductor sealers and the like required to have remarkable electric properties or a curing agent for epoxy resins and also used as a binder for laminated plates, forming materials, mold materials, and the like.
But, when the conventional phenolic novolak was used for the electric and electronic materials, semiconductor sealers or the like, the resin contained a monomeric phenol and a dimeric phenol in a large amount, so that there were various problems resulting from the monomeric phenol such as environmental pollution because of volatilization of the monomeric phenol involving an odor, a degradation in production efficiency because of falling of the dimeric phenol, which were sublimated to deposit on the side and ceiling of the apparatus when a photoresist coated on a substrate such as glass for liquid crystal display was baked, onto the substrate, and a degradation in crosslinking density of a cured resin substance affecting on the heat resistance, and the like. Because the molecular weight distribution was broad, there were also disadvantages that the molten resin had a high viscosity and poor moldability.
Where the phenolic novolak is used as a binder for castings, it is demanded to decrease phenol monomer contained in the phenolic resin as low as possible because the phenol monomer applies to the PRTR method (Pollutant Release and Transfer Register) and the phenol monomer contained in the binder becomes rosin in a high-temperature atmosphere, resulting in defective casting such as a gas defect. In other words, as the phenolic resin for the casting binder, a phenolic novolak having properties that a content of monomeric phenol is decreased, a molecular weight is low and the molecular weight distribution is narrow is demanded.
For example, as a method for reduction of the monomeric phenol and dimeric phenol contained in the above phenolic resin, there is proposed a method of removing low-molecular weight components by blowing in inert gas or vapor to condense a novolak-based condensate at 150° C. to 210° C. after the condensation reaction is completed (Japanese Patent Publication No. Hei 7-91352). This method has a reliable effect of decreasing the monomeric phenol and dimeric phenol but has a disadvantage that their removal lowers the yield of the resin.
The present invention has been made in view of the above circumstances and provides phenolic novolak with the contents of a monomeric phenol and a dimeric phenol and a degree of dispersion (Mw/Mn) controlled.
Specifically, the invention provides phenolic novolak which is free from environmental pollution by a monomeric phenol and a dimeric phenol, inhibition of productivity in a photoresist baking step or a decrease in crosslinking density of a cured resin substance and can have the viscosity of the molten resin lowered, and phenolic novolak which has a small amount of monomeric phenol, a large amount of dimeric phenol, a low molecular weight and a narrow molecular weight distribution.
The present invention also provides a process for production capable of producing the above-described resin in high yield.